1. Field of the Invention
The present invention relates generally to sheet conveying mechanisms in electrophotographic image forming apparatuses, and more particularly to improving precision in correcting the sheet position in the main scanning direction and correcting a skewed condition of a sheet.
2. Description of the Related Art
In image forming apparatuses such as laser printers, sheets such as transfer sheets stacked on a sheet feeder are conveyed one by one. Then, a toner image formed on a photoconductive drum or a photoconductive belt is transferred onto each sheet at a transfer position. Finally, the toner image is fixed onto the sheet, thereby obtaining a recorded sheet.
In such an image forming apparatus, a registration mechanism including a stopper and a pair of rollers is provided just before the transfer position. The registration mechanism corrects the position of a sheet so that the toner image is transferred onto the correct position.
In this image forming apparatus, the stopper is provided on the sheet conveyance path, which stopper determines the position of a sheet in a direction perpendicular to the sheet conveying direction. The leading edge of a sheet abuts the stopper, and while the leading edge is being stopped, a conveying unit positioned on the upstream side conveys the sheet, so that the sheet forms a loop. Then, the stopper is released, so that the leading edge of the sheet is nipped and conveyed by the pair of rollers situated downstream of the stopper. A detecting unit is arranged near a downstream position of the stopper for detecting side portions of the sheet. A moving unit includes a pair of rollers that is movable in a direction orthogonal to the sheet conveying direction. The detecting unit and the moving unit function to correct the sheet position so that the sheet is positioned along a sheet scanning reference position (see, for example, Patent Document 1).
FIG. 7 is a schematic diagram of a conventional sheet conveying mechanism.
In FIG. 7, the reference numeral 32 denotes a pair of horizontal registration rollers, 33 denotes a stopper, 34 denotes a pair of feed rollers, 35 denotes a sheet edge detecting sensor, 36 denotes a pair of conveying rollers, 37 and 38 denote sheet conveyance paths, 39 denotes a sheet, 40 and 41 denote sheet trays, C denotes a buffer, and D denotes a sheet conveyance path junction.
The stopper 33 is arranged at a stage immediately before the pair of horizontal registration rollers 32. The stopper can be switched between a position for closing the sheet conveyance path and a position for opening the sheet conveyance path. The sheet conveyance path is configured in such a manner that the distance between the pair of horizontal registration rollers 32 and the pair of feed rollers 34 is wide enough for a small-sized sheet to be conveyed. Furthermore, there are two sheet conveyance paths at the stage before the pair of feed rollers 34; i.e., the conveyance path 38 extending from the sheet tray 40 provided in the main unit of an image forming apparatus (e.g., a printer) and the conveyance path 37 extending from the sheet tray 41 outside the image forming apparatus. Each of these conveyance paths 37 and 38 is provided with one of the pairs of conveying rollers 36 for sending the sheet 39 toward the pair of feed rollers 34. Furthermore, these two conveyance paths 37 and 38 merge at the junction D located on the upstream side of the pair of feed rollers 34.
Operations of correcting the sheet conveying position and correcting a skewed condition of the sheet 39 are described. The sheet 39 being conveyed by the pair of feed rollers 34 is stopped as the leading edge of the sheet 39 abuts the stopper 33, which stopper 33 is previously situated at a position for closing the sheet conveyance path. At this point, the leading edge of the sheet 39 abuts along the stopper 33, and therefore, a skewed condition of the sheet 39 is corrected. Then, the pair of feed rollers 34 conveys the sheet 39 for a certain amount of time, so that the buffer C is formed between the stopper 33 and the pair of feed rollers 34. Subsequently, the stopper 33 is lowered, thus releasing the leading edge of the sheet 39 from the stopped status. Consequently, due to the rigidity of the buffer C formed in the sheet 39, the leading edge of the sheet 39 is forced to stick out and wedge into the nip portion of the pair of horizontal registration rollers 32. At this point, the sheet 39 is released from the nip of the pair of feed rollers 34, a position of the edge (side edge) of the sheet 39 in the sheet main scanning direction is detected by the sheet edge detecting sensor 35, and the correction amount in the sheet main scanning direction is calculated. Then, the pair of horizontal registration rollers 32 is caused to horizontally move in the roller axial direction in accordance with the calculated correction amount. Accordingly, by performing the operation of correcting the sheet position in the main scanning direction (horizontal registration) with the pair of horizontal registration rollers 32, it is possible to align the position of the sheet 39 with the main scanning direction without affecting the pair of feed rollers 34.
In this sheet conveying mechanism, to correct the sheet conveying position and to correct a skewed condition of a sheet that is longer than the distance between the pair of horizontal registration rollers 32 and the pair of conveying rollers 36, the following situation may occur. That is, the trailing edge of the sheet 39 may still be sandwiched (held with pressure) by the pair of conveying rollers 36 when the leading edge of the sheet 39 has wedged into the nip portion of the pair of horizontal registration rollers 32. In such a condition, if the pair of horizontal registration rollers 32 is horizontally moved to correct the sheet position in the main scanning direction, the nip portion of the pair of conveying rollers 36 will act as a resistance. As a result, the sheet 39 may become twisted and wrinkled, or the skew of the sheet 39 that has been corrected at the stopper 33 may reappear. For these reasons, in this case, the nip portion of the pair of conveying rollers 36 is opened.
Incidentally, when the leading edge of the sheet 39 is released from the stopped status by lowering the stopper 33 after the buffer C has been formed between the stopper 33 and the pair of feed rollers 34, the following situation may occur if the sheet 39 is curled or if the sheet 39 has low rigidity. That is, the sheet 39 may become buckled or skewed before being nipped by the pair of horizontal registration rollers 32, so that the position of the sheet 39 is shifted or a paper jam occurs. Meanwhile, if the sheet 39 is highly rigid, the skew of the sheet 39 corrected at the stopper 33 may reappear before the sheet 39 wedges into the nip portion of the pair of horizontal registration rollers 32. If this happens, it would be meaningless to correct the skew at the stopper 33. To solve these problems, there is a configuration in which the stopper 33 is arranged on the downstream side of the pair of horizontal registration rollers 32 (see, for example, Patent Document 2).
In the above configuration, both the stopper and the conveying unit need to be provided with a driving unit, which leads to an increase in the size of the apparatus as well as higher manufacturing costs.
Even if the above problems are solved, when conveying a thick sheet that has body and that is longer than the distance between the pair of horizontal registration rollers 32 and the sheet conveyance path junction D, a problem arises if the curvature radius of each of the sheet conveyance paths between the corresponding sheet tray and the pair of feed rollers 34 is too small. Specifically, the trailing edge of the sheet remaining in the sheet conveyance path receives a large conveyance resistance that is caused by the small curvature radius of the sheet conveyance path. As a result, the resistance caused by the small curvature radius of the sheet conveyance path obstructs the movement of conveying the sheet 39 in the main scanning direction when correcting the position of the sheet 39 in the main scanning direction with the pair of horizontal registration rollers 32. This decreases the precision of conveying and aligning the sheet.    Patent Document 1: Japanese Patent No. 2893540    Patent Document 2: Japanese Laid-Open Patent Application No. H10-203690